[PATCH 2/2] readahead: sanify file_ra_state names

[PATCH 2/2] readahead: sanify file_ra_state names

[Fengguang Wu]

[-- Attachment #1: readahead-rename.patch --]
[-- Type: text/plain, Size: 8151 bytes --]

Rename some file_ra_state variables and remove some accessors.

It results in much simpler code.
Kudos to Rusty!

Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
---
 include/linux/fs.h |   61 +++-----------------------------------
 mm/readahead.c     |   68 +++++++++++++++----------------------------
 2 files changed, 31 insertions(+), 98 deletions(-)

--- linux-2.6.22-rc4-mm2.orig/include/linux/fs.h
+++ linux-2.6.22-rc4-mm2/include/linux/fs.h
@@ -768,16 +768,12 @@ struct fown_struct {
 
 /*
  * Track a single file's readahead state
- *
- *  ================#============|==================#==================|
- *                  ^            ^                  ^                  ^
- *  file_ra_state.la_index    .ra_index   .lookahead_index   .readahead_index
  */
 struct file_ra_state {
-	pgoff_t la_index;               /* enqueue time */
-	pgoff_t ra_index;               /* begin offset */
-	pgoff_t lookahead_index;        /* time to do next readahead */
-	pgoff_t readahead_index;        /* end offset */
+	pgoff_t start;                  /* where readahead started */
+	unsigned long size;             /* # of readahead pages */
+	unsigned long async_size;       /* do asynchronous readahead when
+					   there are only # of pages ahead */
 
 	unsigned long ra_pages;		/* Maximum readahead window */
 	unsigned long mmap_hit;		/* Cache hit stat for mmap accesses */
@@ -787,59 +783,14 @@ struct file_ra_state {
 };
 
 /*
- * Measuring read-ahead sizes.
- *
- *                  |----------- readahead size ------------>|
- *  ===#============|==================#=====================|
- *     |------- invoke interval ------>|-- lookahead size -->|
- */
-static inline unsigned long ra_readahead_size(struct file_ra_state *ra)
-{
-	return ra->readahead_index - ra->ra_index;
-}
-
-static inline unsigned long ra_lookahead_size(struct file_ra_state *ra)
-{
-	return ra->readahead_index - ra->lookahead_index;
-}
-
-static inline unsigned long ra_invoke_interval(struct file_ra_state *ra)
-{
-	return ra->lookahead_index - ra->la_index;
-}
-
-/*
  * Check if @index falls in the readahead windows.
  */
 static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index)
 {
-	return (index >= ra->la_index &&
-		index <  ra->readahead_index);
-}
-
-/*
- * Where is the old read-ahead and look-ahead?
- */
-static inline void ra_set_index(struct file_ra_state *ra,
-				pgoff_t la_index, pgoff_t ra_index)
-{
-	ra->la_index = la_index;
-	ra->ra_index = ra_index;
+	return (index >= ra->start &&
+		index <  ra->start + ra->size);
 }
 
-/*
- * Where is the new read-ahead and look-ahead?
- */
-static inline void ra_set_size(struct file_ra_state *ra,
-				unsigned long ra_size, unsigned long la_size)
-{
-	ra->readahead_index = ra->ra_index + ra_size;
-	ra->lookahead_index = ra->ra_index + ra_size - la_size;
-}
-
-unsigned long ra_submit(struct file_ra_state *ra,
-		       struct address_space *mapping, struct file *filp);
-
 struct file {
 	/*
 	 * fu_list becomes invalid after file_free is called and queued via
--- linux-2.6.22-rc4-mm2.orig/mm/readahead.c
+++ linux-2.6.22-rc4-mm2/mm/readahead.c
@@ -245,21 +245,16 @@ unsigned long max_sane_readahead(unsigne
 /*
  * Submit IO for the read-ahead request in file_ra_state.
  */
-unsigned long ra_submit(struct file_ra_state *ra,
+static unsigned long ra_submit(struct file_ra_state *ra,
 		       struct address_space *mapping, struct file *filp)
 {
-	unsigned long ra_size;
-	unsigned long la_size;
 	int actual;
 
-	ra_size = ra_readahead_size(ra);
-	la_size = ra_lookahead_size(ra);
 	actual = __do_page_cache_readahead(mapping, filp,
-					ra->ra_index, ra_size, la_size);
+					ra->start, ra->size, ra->async_size);
 
 	return actual;
 }
-EXPORT_SYMBOL_GPL(ra_submit);
 
 /*
  * Set the initial window size, round to next power of 2 and square
@@ -288,7 +283,7 @@ static unsigned long get_init_ra_size(un
 static unsigned long get_next_ra_size(struct file_ra_state *ra,
 						unsigned long max)
 {
-	unsigned long cur = ra->readahead_index - ra->ra_index;
+	unsigned long cur = ra->size;
 	unsigned long newsize;
 
 	if (cur < max / 16)
@@ -305,28 +300,21 @@ static unsigned long get_next_ra_size(st
  * The fields in struct file_ra_state represent the most-recently-executed
  * readahead attempt:
  *
- *                    |-------- last readahead window -------->|
- *       |-- application walking here -->|
- * ======#============|==================#=====================|
- *       ^la_index    ^ra_index          ^lookahead_index      ^readahead_index
- *
- * [ra_index, readahead_index) represents the last readahead window.
- *
- * [la_index, lookahead_index] is where the application would be walking(in
- * the common case of cache-cold sequential reads): the last window was
- * established when the application was at la_index, and the next window will
- * be bring in when the application reaches lookahead_index.
+ *                        |<----- async_size ---------|
+ *     |------------------- size -------------------->|
+ *     |==================#===========================|
+ *     ^start             ^page marked with PG_readahead
  *
  * To overlap application thinking time and disk I/O time, we do
  * `readahead pipelining': Do not wait until the application consumed all
  * readahead pages and stalled on the missing page at readahead_index;
- * Instead, submit an asynchronous readahead I/O as early as the application
- * reads on the page at lookahead_index. Normally lookahead_index will be
- * equal to ra_index, for maximum pipelining.
+ * Instead, submit an asynchronous readahead I/O as soon as there are
+ * only async_size pages left in the readahead window. Normally async_size
+ * will be equal to size, for maximum pipelining.
  *
  * In interleaved sequential reads, concurrent streams on the same fd can
  * be invalidating each other's readahead state. So we flag the new readahead
- * page at lookahead_index with PG_readahead, and use it as readahead
+ * page at (start+size-async_size) with PG_readahead, and use it as readahead
  * indicator. The flag won't be set on already cached pages, to avoid the
  * readahead-for-nothing fuss, saving pointless page cache lookups.
  *
@@ -355,24 +343,21 @@ ondemand_readahead(struct address_space 
 		   unsigned long req_size)
 {
 	unsigned long max;	/* max readahead pages */
-	pgoff_t ra_index;	/* readahead index */
-	unsigned long ra_size;	/* readahead size */
-	unsigned long la_size;	/* lookahead size */
 	int sequential;
 
 	max = ra->ra_pages;
 	sequential = (offset - ra->prev_index <= 1UL) || (req_size > max);
 
 	/*
-	 * Lookahead/readahead hit, assume sequential access.
+	 * It's the expected callback offset, assume sequential access.
 	 * Ramp up sizes, and push forward the readahead window.
 	 */
-	if (offset && (offset == ra->lookahead_index ||
-			offset == ra->readahead_index)) {
-		ra_index = ra->readahead_index;
-		ra_size = get_next_ra_size(ra, max);
-		la_size = ra_size;
-		goto fill_ra;
+	if (offset && (offset == (ra->start + ra->size - ra->async_size) ||
+			offset == (ra->start + ra->size))) {
+		ra->start += ra->size;
+		ra->size = get_next_ra_size(ra, max);
+		ra->async_size = ra->size;
+		goto readit;
 	}
 
 	/*
@@ -391,24 +376,21 @@ ondemand_readahead(struct address_space 
 	 * 	- oversize random read
 	 * Start readahead for it.
 	 */
-	ra_index = offset;
-	ra_size = get_init_ra_size(req_size, max);
-	la_size = ra_size > req_size ? ra_size - req_size : ra_size;
+	ra->start = offset;
+	ra->size = get_init_ra_size(req_size, max);
+	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
 
 	/*
-	 * Hit on a lookahead page without valid readahead state.
+	 * Hit on a marked page without valid readahead state.
 	 * E.g. interleaved reads.
 	 * Not knowing its readahead pos/size, bet on the minimal possible one.
 	 */
 	if (hit_readahead_marker) {
-		ra_index++;
-		ra_size = min(4 * ra_size, max);
+		ra->start++;
+		ra->size = min(4 * ra->size, max);
 	}
 
-fill_ra:
-	ra_set_index(ra, offset, ra_index);
-	ra_set_size(ra, ra_size, la_size);
-
+readit:
 	return ra_submit(ra, mapping, filp);
 }
 

--

[PATCH 0/2] ondemand readahead simplifications

[Fengguang Wu]

Andrew,

With the help from Rusty, the interface and data structure of ondemand
readahead are made more clear:

	readahead: split ondemand readahead interface into two functions
	readahead: sanify file_ra_state names

diffstat:
 fs/ext3/dir.c      |    4 -
 fs/ext4/dir.c      |    4 -
 fs/splice.c        |    6 -
 include/linux/fs.h |   61 +--------------
 include/linux/mm.h |   20 +++--
 mm/filemap.c       |   10 +-
 mm/readahead.c     |  165 +++++++++++++++++++++----------------------
 7 files changed, 116 insertions(+), 154 deletions(-)

Regards,
Fengguang

[PATCH 1/2] readahead: split ondemand readahead interface into two functions

[Fengguang Wu]

[-- Attachment #1: readahead-interface-rusty.patch --]
[-- Type: text/plain, Size: 9276 bytes --]

Split ondemand readahead interface into two functions.  I think this
makes it a little clearer for non-readahead experts (like Rusty).

Internally they both call ondemand_readahead(), but the page argument
is changed to an obvious boolean flag.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
---
 fs/ext3/dir.c      |    4 -
 fs/ext4/dir.c      |    4 -
 fs/splice.c        |    6 +-
 include/linux/mm.h |   20 ++++++---
 mm/filemap.c       |   10 ++--
 mm/readahead.c     |   95 ++++++++++++++++++++++++++-----------------
 6 files changed, 84 insertions(+), 55 deletions(-)

--- linux-2.6.22-rc4-mm2.orig/fs/ext3/dir.c
+++ linux-2.6.22-rc4-mm2/fs/ext3/dir.c
@@ -139,10 +139,10 @@ static int ext3_readdir(struct file * fi
 			pgoff_t index = map_bh.b_blocknr >>
 					(PAGE_CACHE_SHIFT - inode->i_blkbits);
 			if (!ra_has_index(&filp->f_ra, index))
-				page_cache_readahead_ondemand(
+				page_cache_sync_readahead(
 					sb->s_bdev->bd_inode->i_mapping,
 					&filp->f_ra, filp,
-					NULL, index, 1);
+					index, 1);
 			filp->f_ra.prev_index = index;
 			bh = ext3_bread(NULL, inode, blk, 0, &err);
 		}
--- linux-2.6.22-rc4-mm2.orig/fs/ext4/dir.c
+++ linux-2.6.22-rc4-mm2/fs/ext4/dir.c
@@ -138,10 +138,10 @@ static int ext4_readdir(struct file * fi
 			pgoff_t index = map_bh.b_blocknr >>
 					(PAGE_CACHE_SHIFT - inode->i_blkbits);
 			if (!ra_has_index(&filp->f_ra, index))
-				page_cache_readahead_ondemand(
+				page_cache_sync_readahead(
 					sb->s_bdev->bd_inode->i_mapping,
 					&filp->f_ra, filp,
-					NULL, index, 1);
+					index, 1);
 			filp->f_ra.prev_index = index;
 			bh = ext4_bread(NULL, inode, blk, 0, &err);
 		}
--- linux-2.6.22-rc4-mm2.orig/fs/splice.c
+++ linux-2.6.22-rc4-mm2/fs/splice.c
@@ -304,8 +304,8 @@ __generic_file_splice_read(struct file *
 	 * readahead/allocate the rest.
 	 */
 	if (spd.nr_pages < nr_pages)
-		page_cache_readahead_ondemand(mapping, &in->f_ra, in,
-				NULL, index, req_pages - spd.nr_pages);
+		page_cache_sync_readahead(mapping, &in->f_ra, in,
+				index, req_pages - spd.nr_pages);
 
 	while (spd.nr_pages < nr_pages) {
 		/*
@@ -360,7 +360,7 @@ __generic_file_splice_read(struct file *
 		page = pages[page_nr];
 
 		if (PageReadahead(page))
-			page_cache_readahead_ondemand(mapping, &in->f_ra, in,
+			page_cache_async_readahead(mapping, &in->f_ra, in,
 					page, index, req_pages - page_nr);
 
 		/*
--- linux-2.6.22-rc4-mm2.orig/include/linux/mm.h
+++ linux-2.6.22-rc4-mm2/include/linux/mm.h
@@ -1146,12 +1146,20 @@ int do_page_cache_readahead(struct addre
 			pgoff_t offset, unsigned long nr_to_read);
 int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 			pgoff_t offset, unsigned long nr_to_read);
-unsigned long page_cache_readahead_ondemand(struct address_space *mapping,
-			  struct file_ra_state *ra,
-			  struct file *filp,
-			  struct page *page,
-			  pgoff_t offset,
-			  unsigned long size);
+
+void page_cache_sync_readahead(struct address_space *mapping,
+			       struct file_ra_state *ra,
+			       struct file *filp,
+			       pgoff_t offset,
+			       unsigned long size);
+
+void page_cache_async_readahead(struct address_space *mapping,
+				struct file_ra_state *ra,
+				struct file *filp,
+				struct page *pg,
+				pgoff_t offset,
+				unsigned long size);
+
 unsigned long max_sane_readahead(unsigned long nr);
 
 /* Do stack extension */
--- linux-2.6.22-rc4-mm2.orig/mm/filemap.c
+++ linux-2.6.22-rc4-mm2/mm/filemap.c
@@ -913,15 +913,15 @@ void do_generic_mapping_read(struct addr
 find_page:
 		page = find_get_page(mapping, index);
 		if (!page) {
-			page_cache_readahead_ondemand(mapping,
-					&ra, filp, page,
+			page_cache_sync_readahead(mapping,
+					&ra, filp,
 					index, last_index - index);
 			page = find_get_page(mapping, index);
 			if (unlikely(page == NULL))
 				goto no_cached_page;
 		}
 		if (PageReadahead(page)) {
-			page_cache_readahead_ondemand(mapping,
+			page_cache_async_readahead(mapping,
 					&ra, filp, page,
 					index, last_index - index);
 		}
@@ -1382,14 +1382,14 @@ retry_find:
 	 */
 	if (VM_SequentialReadHint(vma)) {
 		if (!page) {
-			page_cache_readahead_ondemand(mapping, ra, file, page,
+			page_cache_sync_readahead(mapping, ra, file,
 							   fdata->pgoff, 1);
 			page = find_lock_page(mapping, fdata->pgoff);
 			if (!page)
 				goto no_cached_page;
 		}
 		if (PageReadahead(page)) {
-			page_cache_readahead_ondemand(mapping, ra, file, page,
+			page_cache_async_readahead(mapping, ra, file, page,
 							   fdata->pgoff, 1);
 		}
 	}
--- linux-2.6.22-rc4-mm2.orig/mm/readahead.c
+++ linux-2.6.22-rc4-mm2/mm/readahead.c
@@ -351,7 +351,7 @@ static unsigned long get_next_ra_size(st
 static unsigned long
 ondemand_readahead(struct address_space *mapping,
 		   struct file_ra_state *ra, struct file *filp,
-		   struct page *page, pgoff_t offset,
+		   bool hit_readahead_marker, pgoff_t offset,
 		   unsigned long req_size)
 {
 	unsigned long max;	/* max readahead pages */
@@ -379,7 +379,7 @@ ondemand_readahead(struct address_space 
 	 * Standalone, small read.
 	 * Read as is, and do not pollute the readahead state.
 	 */
-	if (!page && !sequential) {
+	if (!hit_readahead_marker && !sequential) {
 		return __do_page_cache_readahead(mapping, filp,
 						offset, req_size, 0);
 	}
@@ -400,7 +400,7 @@ ondemand_readahead(struct address_space 
 	 * E.g. interleaved reads.
 	 * Not knowing its readahead pos/size, bet on the minimal possible one.
 	 */
-	if (page) {
+	if (hit_readahead_marker) {
 		ra_index++;
 		ra_size = min(4 * ra_size, max);
 	}
@@ -413,50 +413,71 @@ fill_ra:
 }
 
 /**
- * page_cache_readahead_ondemand - generic file readahead
+ * page_cache_sync_readahead - generic file readahead
  * @mapping: address_space which holds the pagecache and I/O vectors
  * @ra: file_ra_state which holds the readahead state
  * @filp: passed on to ->readpage() and ->readpages()
- * @page: the page at @offset, or NULL if non-present
- * @offset: start offset into @mapping, in PAGE_CACHE_SIZE units
+ * @offset: start offset into @mapping, in pagecache page-sized units
  * @req_size: hint: total size of the read which the caller is performing in
- *            PAGE_CACHE_SIZE units
+ *            pagecache pages
  *
- * page_cache_readahead_ondemand() is the entry point of readahead logic.
- * This function should be called when it is time to perform readahead:
- * 1) @page == NULL
- *    A cache miss happened, time for synchronous readahead.
- * 2) @page != NULL && PageReadahead(@page)
- *    A look-ahead hit occured, time for asynchronous readahead.
+ * page_cache_sync_readahead() should be called when a cache miss happened:
+ * it will submit the read.  The readahead logic may decide to piggyback more
+ * pages onto the read request if access patterns suggest it will improve
+ * performance.
  */
-unsigned long
-page_cache_readahead_ondemand(struct address_space *mapping,
-				struct file_ra_state *ra, struct file *filp,
-				struct page *page, pgoff_t offset,
-				unsigned long req_size)
+void page_cache_sync_readahead(struct address_space *mapping,
+			       struct file_ra_state *ra, struct file *filp,
+			       pgoff_t offset, unsigned long req_size)
 {
 	/* no read-ahead */
 	if (!ra->ra_pages)
-		return 0;
+		return;
 
-	if (page) {
-		/*
-		 * It can be PG_reclaim.
-		 */
-		if (PageWriteback(page))
-			return 0;
-
-		ClearPageReadahead(page);
-
-		/*
-		 * Defer asynchronous read-ahead on IO congestion.
-		 */
-		if (bdi_read_congested(mapping->backing_dev_info))
-			return 0;
-	}
+	/* do read-ahead */
+	ondemand_readahead(mapping, ra, filp, false, offset, req_size);
+}
+EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
+
+/**
+ * page_cache_async_readahead - file readahead for marked pages
+ * @mapping: address_space which holds the pagecache and I/O vectors
+ * @ra: file_ra_state which holds the readahead state
+ * @filp: passed on to ->readpage() and ->readpages()
+ * @page: the page at @offset which has the PG_readahead flag set
+ * @offset: start offset into @mapping, in pagecache page-sized units
+ * @req_size: hint: total size of the read which the caller is performing in
+ *            pagecache pages
+ *
+ * page_cache_async_ondemand() should be called when a page is used which
+ * has the PG_readahead flag: this is a marker to suggest that the application
+ * has used up enough of the readahead window that we should start pulling in
+ * more pages. */
+void
+page_cache_async_readahead(struct address_space *mapping,
+			   struct file_ra_state *ra, struct file *filp,
+			   struct page *page, pgoff_t offset,
+			   unsigned long req_size)
+{
+	/* no read-ahead */
+	if (!ra->ra_pages)
+		return;
+
+	/*
+	 * Same bit is used for PG_readahead and PG_reclaim.
+	 */
+	if (PageWriteback(page))
+		return;
+
+	ClearPageReadahead(page);
+
+	/*
+	 * Defer asynchronous read-ahead on IO congestion.
+	 */
+	if (bdi_read_congested(mapping->backing_dev_info))
+		return;
 
 	/* do read-ahead */
-	return ondemand_readahead(mapping, ra, filp, page,
-					offset, req_size);
+	ondemand_readahead(mapping, ra, filp, true, offset, req_size);
 }
-EXPORT_SYMBOL_GPL(page_cache_readahead_ondemand);
+EXPORT_SYMBOL_GPL(page_cache_async_readahead);

--

[PATCH 2/2] readahead: sanify file_ra_state names

[Fengguang Wu]

[-- Attachment #1: readahead-rename.patch --]
[-- Type: text/plain, Size: 8154 bytes --]

Rename some file_ra_state variables and remove some accessors.

It results in much simpler code.
Kudos to Rusty!

Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
---
 include/linux/fs.h |   61 +++-----------------------------------
 mm/readahead.c     |   68 +++++++++++++++----------------------------
 2 files changed, 31 insertions(+), 98 deletions(-)

--- linux-2.6.22-rc4-mm2.orig/include/linux/fs.h
+++ linux-2.6.22-rc4-mm2/include/linux/fs.h
@@ -768,16 +768,12 @@ struct fown_struct {
 
 /*
  * Track a single file's readahead state
- *
- *  ================#============|==================#==================|
- *                  ^            ^                  ^                  ^
- *  file_ra_state.la_index    .ra_index   .lookahead_index   .readahead_index
  */
 struct file_ra_state {
-	pgoff_t la_index;               /* enqueue time */
-	pgoff_t ra_index;               /* begin offset */
-	pgoff_t lookahead_index;        /* time to do next readahead */
-	pgoff_t readahead_index;        /* end offset */
+	pgoff_t start;                  /* where readahead started */
+	unsigned long size;             /* # of readahead pages */
+	unsigned long async_size;       /* do asynchronous readahead when
+					   there are only # of pages ahead */
 
 	unsigned long ra_pages;		/* Maximum readahead window */
 	unsigned long mmap_hit;		/* Cache hit stat for mmap accesses */
@@ -787,59 +783,14 @@ struct file_ra_state {
 };
 
 /*
- * Measuring read-ahead sizes.
- *
- *                  |----------- readahead size ------------>|
- *  ===#============|==================#=====================|
- *     |------- invoke interval ------>|-- lookahead size -->|
- */
-static inline unsigned long ra_readahead_size(struct file_ra_state *ra)
-{
-	return ra->readahead_index - ra->ra_index;
-}
-
-static inline unsigned long ra_lookahead_size(struct file_ra_state *ra)
-{
-	return ra->readahead_index - ra->lookahead_index;
-}
-
-static inline unsigned long ra_invoke_interval(struct file_ra_state *ra)
-{
-	return ra->lookahead_index - ra->la_index;
-}
-
-/*
  * Check if @index falls in the readahead windows.
  */
 static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index)
 {
-	return (index >= ra->la_index &&
-		index <  ra->readahead_index);
-}
-
-/*
- * Where is the old read-ahead and look-ahead?
- */
-static inline void ra_set_index(struct file_ra_state *ra,
-				pgoff_t la_index, pgoff_t ra_index)
-{
-	ra->la_index = la_index;
-	ra->ra_index = ra_index;
+	return (index >= ra->start &&
+		index <  ra->start + ra->size);
 }
 
-/*
- * Where is the new read-ahead and look-ahead?
- */
-static inline void ra_set_size(struct file_ra_state *ra,
-				unsigned long ra_size, unsigned long la_size)
-{
-	ra->readahead_index = ra->ra_index + ra_size;
-	ra->lookahead_index = ra->ra_index + ra_size - la_size;
-}
-
-unsigned long ra_submit(struct file_ra_state *ra,
-		       struct address_space *mapping, struct file *filp);
-
 struct file {
 	/*
 	 * fu_list becomes invalid after file_free is called and queued via
--- linux-2.6.22-rc4-mm2.orig/mm/readahead.c
+++ linux-2.6.22-rc4-mm2/mm/readahead.c
@@ -245,21 +245,16 @@ unsigned long max_sane_readahead(unsigne
 /*
  * Submit IO for the read-ahead request in file_ra_state.
  */
-unsigned long ra_submit(struct file_ra_state *ra,
+static unsigned long ra_submit(struct file_ra_state *ra,
 		       struct address_space *mapping, struct file *filp)
 {
-	unsigned long ra_size;
-	unsigned long la_size;
 	int actual;
 
-	ra_size = ra_readahead_size(ra);
-	la_size = ra_lookahead_size(ra);
 	actual = __do_page_cache_readahead(mapping, filp,
-					ra->ra_index, ra_size, la_size);
+					ra->start, ra->size, ra->async_size);
 
 	return actual;
 }
-EXPORT_SYMBOL_GPL(ra_submit);
 
 /*
  * Set the initial window size, round to next power of 2 and square
@@ -288,7 +283,7 @@ static unsigned long get_init_ra_size(un
 static unsigned long get_next_ra_size(struct file_ra_state *ra,
 						unsigned long max)
 {
-	unsigned long cur = ra->readahead_index - ra->ra_index;
+	unsigned long cur = ra->size;
 	unsigned long newsize;
 
 	if (cur < max / 16)
@@ -305,28 +300,21 @@ static unsigned long get_next_ra_size(st
  * The fields in struct file_ra_state represent the most-recently-executed
  * readahead attempt:
  *
- *                    |-------- last readahead window -------->|
- *       |-- application walking here -->|
- * ======#============|==================#=====================|
- *       ^la_index    ^ra_index          ^lookahead_index      ^readahead_index
- *
- * [ra_index, readahead_index) represents the last readahead window.
- *
- * [la_index, lookahead_index] is where the application would be walking(in
- * the common case of cache-cold sequential reads): the last window was
- * established when the application was at la_index, and the next window will
- * be bring in when the application reaches lookahead_index.
+ *                        |<----- async_size ---------|
+ *     |------------------- size -------------------->|
+ *     |==================#===========================|
+ *     ^start             ^page marked with PG_readahead
  *
  * To overlap application thinking time and disk I/O time, we do
  * `readahead pipelining': Do not wait until the application consumed all
  * readahead pages and stalled on the missing page at readahead_index;
- * Instead, submit an asynchronous readahead I/O as early as the application
- * reads on the page at lookahead_index. Normally lookahead_index will be
- * equal to ra_index, for maximum pipelining.
+ * Instead, submit an asynchronous readahead I/O as soon as there are
+ * only async_size pages left in the readahead window. Normally async_size
+ * will be equal to size, for maximum pipelining.
  *
  * In interleaved sequential reads, concurrent streams on the same fd can
  * be invalidating each other's readahead state. So we flag the new readahead
- * page at lookahead_index with PG_readahead, and use it as readahead
+ * page at (start+size-async_size) with PG_readahead, and use it as readahead
  * indicator. The flag won't be set on already cached pages, to avoid the
  * readahead-for-nothing fuss, saving pointless page cache lookups.
  *
@@ -355,24 +343,21 @@ ondemand_readahead(struct address_space 
 		   unsigned long req_size)
 {
 	unsigned long max;	/* max readahead pages */
-	pgoff_t ra_index;	/* readahead index */
-	unsigned long ra_size;	/* readahead size */
-	unsigned long la_size;	/* lookahead size */
 	int sequential;
 
 	max = ra->ra_pages;
 	sequential = (offset - ra->prev_index <= 1UL) || (req_size > max);
 
 	/*
-	 * Lookahead/readahead hit, assume sequential access.
+	 * It's the expected callback offset, assume sequential access.
 	 * Ramp up sizes, and push forward the readahead window.
 	 */
-	if (offset && (offset == ra->lookahead_index ||
-			offset == ra->readahead_index)) {
-		ra_index = ra->readahead_index;
-		ra_size = get_next_ra_size(ra, max);
-		la_size = ra_size;
-		goto fill_ra;
+	if (offset && (offset == (ra->start + ra->size - ra->async_size) ||
+			offset == (ra->start + ra->size))) {
+		ra->start += ra->size;
+		ra->size = get_next_ra_size(ra, max);
+		ra->async_size = ra->size;
+		goto readit;
 	}
 
 	/*
@@ -391,24 +376,21 @@ ondemand_readahead(struct address_space 
 	 * 	- oversize random read
 	 * Start readahead for it.
 	 */
-	ra_index = offset;
-	ra_size = get_init_ra_size(req_size, max);
-	la_size = ra_size > req_size ? ra_size - req_size : ra_size;
+	ra->start = offset;
+	ra->size = get_init_ra_size(req_size, max);
+	ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size;
 
 	/*
-	 * Hit on a lookahead page without valid readahead state.
+	 * Hit on a marked page without valid readahead state.
 	 * E.g. interleaved reads.
 	 * Not knowing its readahead pos/size, bet on the minimal possible one.
 	 */
 	if (hit_readahead_marker) {
-		ra_index++;
-		ra_size = min(4 * ra_size, max);
+		ra->start++;
+		ra->size = get_next_ra_size(ra, max);
 	}
 
-fill_ra:
-	ra_set_index(ra, offset, ra_index);
-	ra_set_size(ra, ra_size, la_size);
-
+readit:
 	return ra_submit(ra, mapping, filp);
 }
 

--

Re: [PATCH 2/2] readahead: sanify file_ra_state names

[Rusty Russell]

On Thu, 2007-06-14 at 14:21 +0800, Fengguang Wu wrote:
> plain text document attachment (readahead-rename.patch)
> Rename some file_ra_state variables and remove some accessors.
> 
> It results in much simpler code.
> Kudos to Rusty!
> 
> Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>

Needless to say, I'm now very satisfied with the new readahead code.  

That's not to say there won't be corner cases where the old code did
better, but given the replacement's simplicity and Fengguang's positive
benchmark results I really like it.

Thanks for Fengguang for coding and pushing this!
Rusty.